Many hybridization experiments are analyzed by fitting the time curve of per cent of DNA or RNA hybridized to various forms of the second order rate equation. Empirical correction factors must be applied for proper interpretation of results. Since the precise assumptions on which this method of analysis are based have not always been made clear, there is some uncertainty about applicability of particular equations. Also some laboratories lack sufficiently general curve fitting programs. The computer system MLAB, available at NIH and some other installations has been recommended for its power and generality in curve fitting and model building. One simple but important question under what ranges of initial tracer to driver concentration are two of the commonly used approximations to the second prior rate equation valid has been answered by computing and displaying the appropriate curves over a large range of initial concentrations. Improving the model by finding a theoretical basis for the empirical correction factors is the next goal of this project.